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Research Papers

Modeling of Overpotentials in an Anode-Supported Planar SOFC Using a Detailed Simulation Model

[+] Author and Article Information
Henning Severson1

Department of Mechanical and Structural, Engineering and Materials Science,  University of Stavanger, 4036 Stavanger, Norwayhenning.severson@uis.no

Mohsen Assadi

Department of Mechanical and Structural, Engineering and Materials Science,  University of Stavanger, 4036 Stavanger, Norway

1

Corresponding author.

J. Fuel Cell Sci. Technol 8(5), 051021 (Jul 12, 2011) (13 pages) doi:10.1115/1.4004018 History: Received December 01, 2010; Revised April 02, 2011; Published July 12, 2011; Online July 12, 2011

In this paper results from a parameter study of an anode-supported solid oxide fuel cell (SOFC) are presented. The effects on performance, current-voltage (I-V) characteristics, polarization voltages, and diffusion coefficients are modeled for different temperatures, electrolyte thickness, porosities, and pore sizes. The analysis is carried out for a planar SOFC with YSZ electrolyte, LSM cathode, and Ni-YSZ anode, with thicknesses 20, 50, and 500 μm respectively, and with co-flow geometry. The predicted performance is validated with measured data found in the literature with good agreement. Standard equations for binary and Knudsen diffusion in porous media, concentration overpotentials, and Ohm’s law are used in the modeling. Activation overpotential is predicted by use of temperature dependent linear equations at both the anode and cathode sides. It is found that both ohmic and activation overpotentials decrease considerably with increasing temperature, while concentration overpotentials increase moderately with increasing temperature. The effect on concentration overpotentials can be explained by the reduced gas density with increased temperature, despite the increasing diffusion coefficient. Furthermore, it was found that increasing the pore sizes decreases concentration overpotentials. At low pore size the Knudsen diffusion coefficient is a bottleneck for the diffusion coefficient since it is much lower than the binary diffusion coefficient. It has been demonstrated that by increasing the pore size the Knudsen diffusion coefficient is improved. The effect of porosity has much in common with the effect of pore size; increasing porosity leads to decreased concentration overpotential due to the improved diffusion coefficient. As a natural phenomenon for anode-supported cells, most of the concentration overpotentials take place at the anode side due to its thick structure despite the high diffusion coefficient of hydrogen. It must be underlined that in this study the effect of different porosities and pore sizes is modeled at the anode and cathode substrates only without taking into account the length of the three phase boundary (LTPB ) near the electrolyte interface. However, since these microstructural parameters can have an impact on the LTPB , they can also have an impact on the activation overpotentials. This is not considered here and will be taken into account in future work.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 4

Effect of fuel flow on power density at 2.5, 5.0, 7.5, and 10.0 mol h−1 of fuel

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Figure 5

Effect of fuel flow on activation overpotentials at 2.5, 5.0, 7.5, and 10.0 mol h−1 of fuel

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Figure 6

Effect of fuel flow on concentration overpotentials at 2.5, 5.0, 7.5, and 10.0 mol h−1 of fuel

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Figure 7

Effect of fuel flow on ohmic overpotentials at 2.5, 5.0, 7.5, and 10.0 mol h−1 of fuel

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Figure 8

Effect of fuel utilization on I-V characteristics at 50%, 70%, and 85% FU

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Figure 9

Effect of temperature on I-V characteristics at 800°C, 700°C, and 600°C

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Figure 10

Effect of temperature on power density at 800°C, 700°C, and 600°C

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Figure 11

Effect of temperature on activation overpotentials at 800°C, 700°C, and 600°C

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Figure 12

Effect of temperature on concentration overpotentials at 800°C, 700°C, and 600°C

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Figure 13

Effect of temperature on ohmic overpotentials at 800°C, 700°C, and 600°C

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Figure 14

Effect of temperature on effective diffusion coefficients at anode and cathode

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Figure 15

Effect of electrolyte thickness on I-V characteristics

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Figure 16

Effect of electrode porosity on I-V characteristics

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Figure 17

Effect of electrode porosity on concentration overpotentials

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Figure 18

Effect of electrode porosity on effective diffusion coefficients at anode and cathode

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Figure 19

Effect of pore diameter on I-V characteristics

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Figure 21

Effect of pore diameter on effective diffusion coefficients at anode and cathode

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Figure 3

Effect of fuel flow on I-V characteristics at 2.5, 5.0, 7.5, and 10.0 mol h−1 of fuel

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Figure 1

Basic principle of an anode-supported SOFC with co- or counter-flow geometry. ACL and CCL: Anode and cathode catalyst layers.

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Figure 2

Comparison of model predictions and experimental results of Zhao and Virkar [16]

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Figure 20

Effect of pore diameter on concentration overpotentials

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